1. Field of the Invention
This invention relates to the field of night vision optics. More particularly, the invention pertains to testing for insidious degradation in intensity of images seen through night vision goggles of the type used by aviators.
2. Description of the Prior Art
To see at night images located outside an aircraft, aviators use night vision goggles. To see instrument dials located in the cockpit, some illumination in the cockpit is necessary. However, in former night vision systems, if cockpit illumination were intense enough to permit viewing instrument dials at night, it reduced or prevented clear external vision through the goggles. In using former systems, when cockpit light was reduced enough to permit clear external night vision through the goggles, cockpit instruments could not be seen unless the goggles were removed.
To overcome this difficulty and permit acceptable internal and external vision, now filters are used to view external images and colored light, having wavelengths corresponding to a portion of the visible spectrum, is used within the cockpit.
Light from the night sky is known to have wavelengths in the visible range near that of infrared. Accordingly, the aircraft crew now wear goggles at night having a filter through which external images are viewed. The filter passes light having wavelengths in the visible spectrum corresponding to that of light from the night sky and blocks light having shorter wavelengths. Therefore, blue, green, and even yellow light is invisible through the filter. A filter having this capability is part of the Aviator Night Vision System known as ANVIS.